Data communication networks can be categorized as handling transfers of data on either a circuit-switched basis or a packet-switched basis. Where two users want to exchange data in a network which utilizes circuit switching, a path must be established through the network before the data exchange can begin. Once the path is set up, it continues to exist for the duration of the data exchange between the users. If the same two users were connected through a packet-switched network, it would not be necessary to set up and maintain a "circuit" between them. In a packet-switched network, user data is formatted in discrete data units or packets, each of which contains the routing information needed by intermediate systems or nodes to transfer the packet toward its intended destination over currently available links.
A type of packet-switching technology that is becoming increasingly pervasive is Asynchronous Transfer Mode (ATM) technology. In ATM networks, user data is formatted in fixed length cells, each of which includes a header field and a data field. The standard header field is five bytes in length and contains all necessary control and routing information for allowing the cell to be switched through the network toward its destination. The standard data field is forty-eight bytes long. The use of fixed length cells permits much of the necessary switching within the network to be carried out using specialized, high-speed hardware switches.
Users of any kind of data communication network, and not just ATM networks, are often concerned about concealing their data from eavesdroppers (sometimes called interlopers) on the network. Considerable time and effort has been spent developing cryptographic techniques which permit original data (sometimes referred to as plaintext or cleartext) to be encrypted or "scrambled" before it is transmitted as "ciphertext" through the network and then decrypted or returned to its original or plaintext form once it reaches the intended destination. Many encryption techniques employ "keys", which are values that control the encryption and decryption processes.
An illustration of an extremely simple, and largely ineffective, encryption approach is to replace each plaintext character in a message by a character "n" positions away in the alphabet, wrapping or returning to the beginning of the alphabet where the plaintext character is within "n" positions of the end of the alphabet. For example, if n=3, the plaintext word "safe" would translate to the ciphertext word "vdih". In this example, 3 would be considered the encryption key. As long as the party receiving the ciphertext message knows the encryption method and the key, recovery of the plaintext message is relatively simple.
Any effort by an eavesdropper to recover plaintext from an encrypted message is referred to as an "attack" on the message. Just as there are different kinds of encryption, there are different kinds of attacks aimed at discovering the key used to encrypt the plaintext data. Where a user must select the encryption key, it is human nature for that user to select an easily remembered key, such the user's own last name or the name of a favorite hobby; e.g., "golf" or "sailing". Eavesdroppers can take advantage of human nature by employing a "dictionary attack" in which names, English words (for example, all of the words in an unabridged dictionary), birthdays, etc. are tried as decryption keys to see if plaintext is generated. Where an eavesdropper knows the names of the sending and/or receiving parties or the time of transmission of the ciphertext message or other transmission-related information, a "traffic analysis attack" may be mounted by using such information in an effort to find the encryption key. More detailed information about the subject of cryptography is available from a number of references, including the book Bruce Schneier,"Applied Cryptography--Protocols, Algorithms and Source Code in C", John Wiley & Sons (19994). While the present invention is intended for use in networks in which cryptography is practiced, the invention can be understood without requiring any information from this book
Where data is transmitted in fixed length cells, concerns about the security of encrypted data are heightened where the data field in a given cell contains relatively little data, for example, a single character response to an inquiry. Known types of attacks, such as dictionary attacks or traffic analysis attacks, can often be more effectively applied where the attacked cell has a relatively low data content.